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RFC 1583 - 9.3. The Interface state machine

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### 9.3. The Interface state machine

A detailed description of the interface state changes follows. Each state change is invoked by an event (Section 9.2). This event may produce different effects, depending on the current state of the interface. For this reason, the state machine below is organized by current interface state and received event. Each entry in the state machine describes the resulting new interface state and the required set of additional actions.

When an interface's state changes, it may be necessary to originate a new router links advertisement. See Section 12.4 for more details.

Some of the required actions below involve generating events for the neighbor state machine. For example, when an interface becomes inoperative, all neighbor connections associated with the interface must be destroyed. For more information on the neighbor state machine, see Section 10.3.

             State(s):  Down

                Event:  InterfaceUp

            New state:  Depends upon action routine

               Action:  Start the interval Hello Timer, enabling the
                        periodic sending of Hello packets out the interface.
                        If the attached network is a physical point-to-point
                        network or virtual link, the interface state
                        transitions to Point-to-Point.  Else, if the router
                        is not eligible to become Designated Router the
                        interface state transitions to DR Other.

                        Otherwise, the attached network is multi-access and
                        the router is eligible to become Designated Router.
                        In this case, in an attempt to discover the attached
                        network's Designated Router the interface state is
                        set to Waiting and the single shot Wait Timer is
                        started.  If in addition the attached network is
                        non-broadcast, examine the configured list of
                        neighbors for this interface and generate the
                        neighbor event Start for each neighbor that is also
                        eligible to become Designated Router.

             State(s):  Waiting

                Event:  BackupSeen

            New state:  Depends upon action routine.

               Action:  Calculate the attached network's Backup Designated
                        Router and Designated Router, as shown in Section
                        9.4.  As a result of this calculation, the new state
                        of the interface will be either DR Other, Backup or
                        DR.

             State(s):  Waiting

                Event:  WaitTimer

            New state:  Depends upon action routine.

               Action:  Calculate the attached network's Backup Designated
                        Router and Designated Router, as shown in Section
                        9.4.  As a result of this calculation, the new state
                        of the interface will be either DR Other, Backup or
                        DR.

             State(s):  DR Other, Backup or DR

                Event:  NeighborChange

            New state:  Depends upon action routine.

               Action:  Recalculate the attached network's Backup Designated
                        Router and Designated Router, as shown in Section
                        9.4.  As a result of this calculation, the new state
                        of the interface will be either DR Other, Backup or
                        DR.

             State(s):  Any State

                Event:  InterfaceDown

            New state:  Down

               Action:  All interface variables are reset, and interface
                        timers disabled.  Also, all neighbor connections
                        associated with the interface are destroyed.  This
                        is done by generating the event KillNbr on all
                        associated neighbors (see Section 10.2).

             State(s):  Any State

                Event:  LoopInd

            New state:  Loopback

               Action:  Since this interface is no longer connected to the
                        attached network the actions associated with the
                        above InterfaceDown event are executed.

             State(s):  Loopback

                Event:  UnloopInd

            New state:  Down

               Action:  No actions are necessary.  For example, the
                        interface variables have already been reset upon
                        entering the Loopback state.  Note that reception of
                        an InterfaceUp event is necessary before the
                        interface again becomes fully functional.

Next: 9.4. Electing the Designated Router

Connected: An Internet Encyclopedia
RFC 1583 - 9.3. The Interface state machine